Airborne Brake Wear Particle Emissions: A Review

A steady growth in the type and number of transportation vehicles is bound to have profound impacts on environmental protection. Brake wear debris, generating from transportation vehicles during friction braking, has become an important contribution source of airborne brake wear particles and significantly affects the air quality in cites. In order to implement the concept of green development and explore measures to control airborne particle emissions during friction braking, it is necessary to have a thorough understanding of airborne brake wear particle emissions. Hence, the recent progress in airborne particles, generating from friction fair materials during friction braking, was summarized in this study. Firstly, the evolution of wear debris during braking was introduced from the angle of mechanism. The formation mechanism of the friction layer, and the generation and emission characteristics of brake wear debris were briefly described. Secondly, the effects of brake structure, material types, external environment, braking speed and applied load on airborne brake wear particle emissions were elaborated, and the influences of these factors on the emission characteristics (such as particle morphology, particle size and particle concentration) of airborne particles during braking were expounded. Finally, several aspects of research on airborne brake wear particle emissions were proposed.　Airborne particles, causing harmful effects such as carcinogenesis and respiratory disease in men, are the primary source of air pollution in cities. Traffic-generated emissions, arising from exhaust and non-exhaust sources, are one of the major contributors to airborne particulate matter. As a consequence of progressive reductions in exhaust emissions, the relative importance of non-exhaust traffic-related emissions (such as those from brake wear, road wear and tyre wear) has increased. Among them, brake wear particles account for a large part of non-exhaust particle emissions in urban areas. Evaluation of airborne particle emissions during braking is helpful to assess the degree of air pollution, and provide the experimental basis and theoretical guidance for limiting and controlling airborne particle emissions during friction braking. Additionally, the study helps to promote the development of green tribology.　This work expounded upon the derivation and emission process of airborne brake wear particles from the perspective of tribology, summarized the influences of various factors on the formation and emission of airborne brake wear particles, and discussed the formation and emission mechanism of airborne brake wear particles in detail. The trends of the formation and emission of airborne brake wear particles were comprehensively elaborated, owing to the changes of brake structure, material types, external environment, braking speed and applied load. Based on the research findings of the wear mechanism, the formation and emission behavior of airborne particles during braking were described. In addition, it also introduced the recent research results of domestic and foreign scholars on airborne particle emissions during braking, and summarized the laws of the formation and emission characteristics of airborne brake wear particles. Finally, this work reiterated the importance of airborne brake wear particles in tribological research, and proposed the development trend of airborne brake wear particles.　Brake wear debris is an inevitable product during braking process, it participates in the formation of the friction layer or is freely discharged into the atmosphere, depending on its physicochemical properties and movement state. A lot of research on the formation of the friction layer and the emission of brake wear debris has been done in recent years. The formation of the friction layer is mainly associated with the adhesion phenomenon and friction thermal effect of friction surfaces. The adhesion phenomenon induces material transfer between the sliding surfaces during braking. The accumulation of frictional heat, commonly occurring near the hard phase of friction surfaces, are sufficient to activate the formation of the friction layer and the tribo-chemical reactions of the surface materials. The movement trace determines the emission characteristic of brake wear debris, it is can be easy to escape from the friction surface when its initial direction is toward the edge of the friction pair. Several factors affect both physicochemical characteristics and generation rates of airborne brake wear particles. The structure of the brake significantly influence airborne brake wear particle emissions. In general, the emissions are higher from disc brakes than drum brakes for a given set of conditions. The transition in the wear mechanism of friction pair materials depends on material types, external environment and braking parameters, resulting in the different emission characteristics of airborne wear particles originating from friction braking.　Air pollution has been increasingly associated with diverse adverse health outcomes. Brake wear debris, generating from transportation vehicles during friction braking, has become the primary source of air pollution in most cities. Airborne brake wear particle emissions have been studied and characterized, while some of the technological improvements have resulted in a reduction of their emissions in some developed countries. However, in China, their emissions have not yet been adequately studied, and several questions regarding emission characteristics, emission factors and possible adverse health effects of airborne brake wear particles still remain unanswered. Hence, China scholars should pay attention to the environmental impacts of airborne brake wear particles, thoroughly investigate emission characteristics of airborne brake wear particles, systematically analyze the formation and emission mechanism of airborne brake wear particles, and explore measures to reduce and control their emissions.